Silicon nanoclusters: Ultra high vacuum laser ablation fabrication and in situ scanning tunneling microscopy characterization

Nanoscale clusters are very small aggregates of material, composed of tens to several thousands of atoms. Within this size regime, novel properties exist that are intermediate between those of atomic/molecular and bulk systems. Due to the small size of clusters, a large proportion of their atoms can reside on the surface and have a significant effect on cluster stability, reactivity, and quantum properties. Unfortunately, the atomic surface structure of clusters is largely unknown, and their characterization remains an important goal. This work involves the development of an ultra high vacuum laser ablation system for fabrication of unpassivated, substrate supported silicon clusters, with subsequent in situ surface characterization using scanning tunneling microscopy techniques. Images are presented that show clusters with a flattened, irregular morphology, and indicate the formation of intramolecular features, or motifs, within the clusters. Atomic resolution detail is achieved, and used to demonstrate small scale instability of the clusters. The inert nature of the graphite substrate appears to allow significant surface diffusion of material, and likely contributes to cluster instability, as well.